On 11/13/2014 10:27 AM, Richard Earnshaw wrote:
On 13/11/14 17:05, Ramana Radhakrishnan wrote:
On Thu, Nov 13, 2014 at 4:55 PM, Sandra Loosemore
<san...@codesourcery.com> wrote:
This patch to the AArch64 back end adds a couple of additional bics patterns
to match code of the form
if ((x & y) == x) ...;
This is testing whether the bits set in x are a subset of the bits set in y;
or, that no bits in x are set that are not set in y. So, it is equivalent
to
if ((x & ~y) == 0) ...;
Presently this generates code like
and x21, x21, x20
cmp x21, x20
b.eq c0 <main+0xc0>
and this patch allows it to be written more concisely as:
bics x21, x20, x21
b.eq c0 <main+0xc0>
Since the bics instruction sets the condition codes itself, no explicit
comparison is required and the result of the bics computation can be
discarded.
Regression-tested on aarch64-linux-gnu. OK to commit?
Is this not a duplicate of
https://gcc.gnu.org/ml/gcc-patches/2014-11/msg00943.html ?
I don't think so. However, I think it is something that should be
caught in generic simplification code
ie map ((a & b) == b) ==> ((~a & b) == 0), etc
Bit-clear operations are not that uncommon. Furthermore, A may be a
constant.
Alex posted his patch when I already had Chris's in my regression test
queue, but I've just confirmed that it does not fix the test case I
included.
I already thought a little about making this a generic simplification,
but it seemed to me like it was only useful on targets that have a
bit-clear instruction that happens to set condition codes, and that it
would pessimize code on targets that don't have a bit-clear instruction
at all (by inserting the extra complement operation). So to me it
seemed reasonable to do it in the back end.
-Sandra
